An accidental release of 100 to 200 m³ of iron oxide-charged effluents in a thermal power plant in one of its port basins. The release was detected during a security round (brown colouration of the water). During the shutdown of a plant unit, the combustion air heaters of the steam generators were being cleaned as they had become clogged by flying ash deposits. This operation is conduced every two years. For this purpose, the heater’s inspection hatches are opened and the equipment is cleaned with water sprayed at high pressure (no cleaning products are used). The effluents are exceptionally conveyed to an internal tank (SNM) prior to their elimination in the effluent treatment station of the smoke desulphurisation installation. The tank is equipped with 2 screw conveyors which, during normal operation, drain off the overflow from the residual tank to the tank on the port.

During the works, the screws should have been locked out (off), which was not the case. Several organisational malfunctions were noted: lock-out was requested for the following day; an interface problem between the 2 structures concerned was noted (shutdown and lock-out management); the job site was under maintenance’s responsibility although the effluents were monitored by another department; continuous operations at the job site (night/day) with rounds performed only during the day (thus the delay in detection).